26.2 - Identifying aldehydes and ketones

Question 1

What is the solution used to detect the presence of carbonyl compounds?

Answer 1

Brady’s Reagent(dissolved in methanol and sulfuric acid since friction or a sudden blow can cause it to explode) : 2,4-DNP

Question 2

Describe the test for the carbonyl group in aldehydes and ketones. (4)

Answer 2

1. Add 5cm3 depth of a solution of 2,4-dinitrophenylhydrazine to a clean test-tube in excess
2. Using a dropping pipette, add three drops of the unknown compound and leave to stand
3. If no crystals form, add a few drops of sulfuric acid
4. A yellow/orange precipitate indicates the presence of an aldehyde or ketone

Question 3

What is the solution used to detect the presence of aldehydes?

Answer 3

Tollens’ Reagent: solution of silver nitrate in aqueous ammonia

Question 4

Describe the making of Tollens’ Reagent(3)

Answer 4

1. In a clean test-tube, add 3cm depth of aqueous silver nitrate, AgNO3(aq)
2. Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide, Ag2O, is formed
3. Add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution(Tollens’ reagent)

Question 5

Describe the test for aldehydes (3)

Answer 5

1. Pour 2cm depth of the unknown solution into a clean test tube
2. Add an equal volume of freshly prepared Tollens’ Reagent(short half-life)
3. Leave the test-tube to stand in a beaker of warm water (50 degrees) for about 10-15 minutes and then observe whether any silver mirror is formed

Question 6

What type of reaction takes place between an aldehyde and silver ions?

Answer 6

A redox reaction

Question 7

What gets reduced?

Answer 7

Ag+(aq) + e- -> Ag(s)

Question 8

What gets oxidised?

Answer 8

aldehyde -> alcohol

Question 9

Describe how you would identify an aldehyde or ketone by melting point(4)

Answer 9

1. The impure yellow/orange solid is filtered to separate the solid precipitate from the solution
2. The solid is recrystalised to produce a pure sample of crystals
3. The melting point of 2,4-DNP is measured and recorded
4. The melting point is then compared to a database/table to identify the original carbon compound